Modern multi-carrier communication systems such as the Universal Mobile Telecommunications System (UMTS) and the Long Term Evolution (LTE) system standardized by the 3rd Generation Partnership Project (3GPP) are configured to provide transmission gaps on a serving carrier. The transmission gaps may be exploited by a device served on the serving carrier to temporarily tune to another carrier for cell search procedures, signal power measurements and other purposes. During the transmission gaps, no dedicated signalling is performed on the serving carrier in relation to the served device, and no signalling on the serving carrier is expected from the served device.
When, for example, the device is served on a UMTS Terrestrial Radio Access Network (UTRAN) carrier and intends to conduct measurements on an Evolved UTRAN (E-UTRAN) carrier, it is guaranteed a transmission gap (or “measurement gap”) of at least 6.6 ms, with a repetition period of up to 120 ms. For the transmission gap the explicit purpose “E-UTRAN measurement” will be signalled on the serving carrier to the served device.
When the device is served on an E-UTRAN carrier and intends to conduct measurements on another carrier (of the same or a different Radio Access Technology, RAT), it is guaranteed a measurement gap of 6 ms. In this case, no measurement purpose is signalled. Instead, the device has to schedule its measurement activities dependent on the number of carriers to be investigated. Up to 7 frequency layers may have to be monitored to this end by the device besides the one for the serving cell (intra-carrier), including carriers at other frequencies (inter-carriers) and carriers of a different RAT (inter-RAT carriers).
Detection of E-UTRAN cells is facilitated by synchronization signals transmitted on the Primary Synchronization Channel (P-SCH) and the Secondary Synchronization Channel (S-SCH) every 5 ms. Moreover, Reference Symbol Received Power (RSRP) measurements may be performed for cell detection or other purposes based on reference symbols transmitted in at least some of the sub-frames.
FIG. 1 exemplarily illustrates the timing of synchronization signals and reference symbols in a Frequency Division Duplex (FDD) mode of an E-UTRAN (only the central 72 sub-carriers are shown). As indicated in FIG. 1, some of the sub-frames may be used for Multimedia Broadcast Multicast Service (MBMS) over a Single Frequency Network (MBSFN) transmission and may thus not contain cell-specific reference symbols. FIG. 2 exemplarily illustrates the transmission of synchronization signals and reference symbols in a Time Division Duplex (TDD) mode of an E-UTRAN (again only the central 72 sub-carriers are shown). As indicated in FIG. 2, some of the sub-frames or parts thereof may be used as Guard Periods (GPs) and for UpLink (UL) transmission, and may thus not contain any cell-specific reference symbols.
Since the synchronization signals transmitted in an E-UTRAN have a repetition period of 5 ms, approximately 5.12 to 5.35 ms of effective radio time is required during a transmission gap to reliably detect a cell that has an arbitrary frame timing. It is readily apparent that a longer effective radio time also permits more accurate RSRP measurements because more reference symbols will become available. In an E-UTRAN with transmission gaps of 6 ms the effective radio time is, however, often less than 5 ms due to the time required for tuning the radio front end to the carrier of interest and the AGC settling time. Such a situation is depicted in FIG. 3.
In the situation illustrated in FIG. 3, cells with unfavourable frame timing will be impossible to detect with conventional E-UTRAN cell search procedures. Moreover, the accuracy of RSRP measurements will be lowered in cases in which tuning to the carrier of interest and settling of the AGC takes long. For example, if there is a large difference between the expected signal strength and the actual signal strength, the gain correction of AGC will generally consume a larger portion of the transmission gap than in a scenario with a small difference.
In order to extend the effective radio time for a given transmission gap duration, one could think of representing the received signal samples with significantly more bits such that information loss is prevented in the case of inaccurate gain settings. Additionally, the dynamic range of analog receiver parts may be increased. However, these approaches would require more memory resources and a more sophisticated receiver design, and would thus lead to increased cost.
EP 1 583 232 A2 discloses an AGC device and an AGC method for allowing the device to quickly converge and become stabilized when the tuning frequency of a radio receiver is switched a plurality of times in a short period of time as in a case of frequency monitoring using the compressed mode.